Cursed Piper Flute

Transformed, you fit through single block gaps and you can grip the sides of walls, climbing up or down by holding the direction. You slide while climbing unless you are wearing Climbing Claws or Shoe Spikes, which turns a fiddly mechanic into a controllable one. Speed is roughly what basic boots give you, and long falls barely register.

Using any item drops the transformation instantly, so this is pure movement. That restriction has one exploit built into it: your hitbox while transformed is tiny, and mounting for a moment in the middle of a dense wave of projectiles is a legitimate dodge. Getting back out is one keypress, so the window is short but real.

If you try to dismount somewhere you no longer fit, the game shoves you to the nearest open space after a couple of failed attempts, and to your spawn point if there is nothing close enough. Keep that in mind before squeezing deep into sealed rock.

Ivy Chests, which puts it in the Jungle. The name points at the Pied Piper story, and Reflective Gold Dye turns the rat into a passable Gold Mouse.